In order to reduce variation in threshold voltage due to statistical fluctuation in channel impurity, a non-doped epitaxial silicon layer is formed over a highly doped channel layer that has a steep distribution of impurity density. It is proposed to control the threshold voltage of a transistor by adjusting the thickness of the non-doped epitaxial silicon layer. See, for example, Japanese Laid-open Patent Publication No. 2012-79746. An epitaxial silicon layer formed in an area assigned to high-threshold-voltage transistors is made thinner than an epitaxial silicon layer formed in an area assigned to low-threshold-voltage transistors. With this arrangement, different types of transistors with different performances can be arranged on the same substrate.
FIG. 1 illustrates a conventional process for forming epitaxial silicon layers with different thicknesses. An epitaxial silicon layer 112 is formed over a highly-doped layer 111 on a silicon substrate 131 (S101). Then, a photoresist mask 113 is formed by a photolithographic technique (S102) and a part of the epitaxial silicon layer 112 is etched to produce an epitaxial silicon layer 114 with a different thickness (S103). Then, a first gate oxide film (Gox1) 116 is formed over the entire surface including a trench 115 to form a device-isolating layer 117 (S104). A photoresist layer 123 with an opening pattern in an area not illustrated is formed and an unnecessary portion of the first gate oxide film 116 is removed by etching (S105). This step is referred to as “oxide film etching 1”. Then, the photoresist layer 123 is removed and second gate oxidation is performed to form a second gate oxide (Gox2) film (S106). In an area having been covered with the photoresist layer 123 during the process of oxide film etching 1, a gate oxide layer (Gox1+Gox2) 119 is formed. In the area (not illustrated) to which the oxide film etching 1 was applied, a gate oxide film that is thinner than the gate oxide film 119 is formed. Unnecessary portions of the gate oxide film 119 are removed (S107). Then, a third gate oxide film (Gox3) 24 is formed by gate oxidation 3 (S108).
The conventional method partially changes the thickness of the epitaxial silicon layer using a photolithography technique and an etching technique. Accordingly, at least two additional steps, i.e., a photography step and an etching step are added. It is desirable to create non-doped epitaxial layers with different thicknesses in an ordinary process without adding extra steps.